This invention relates generally to cardiovascular catheters of the type that include optical fibers for spectrometric determination of oxygen saturation or other chemical parameters of blood, and more particularly to a disposable calibration medium and shipping sleeve for such a catheter.
2. Prior Art
It has been recognized for some decades that such cardiovascular catheters of the optical type require calibration to compensate for variations in several parameters--particularly including transmission efficiency of fibers at multiple wavelengths, distance between fibers at the catheter tip, efficiency of optical connectors, and variations in gain of the detectors receiving back-scattered light from the fibers.
Therefore it is necessary to match a catheter to an electrooptical instrument, and calibrate the overall system as a unit. The catheter is connected to the instrument and the catheter tip exposed to a reference medium providing a known amount of back-scatter reflection; the instrument is calibrated; and the catheter then is separated from the reference medium and inserted into the fluid (usually blood) to be measured.
The historical technique for accomplishing the calibration step is to immerse the catheter tip into a reference liquid that contains suspended particles to reflect light back into the catheter. A standard used since the 1950s has been milk of magnesia--that is, a suspension of magnesium oxide.
This technique is subject to disadvantages. It is inconvenient to the user. It places severe sterility requirements on the calibrating liquid. It is hazardous to the patient or research subject, due to potential toxicity of traces of the liquid on the catheter when the catheter is subsequently placed in a living body. All these problems are attributable to the use of a liquid as the standard.
To avoid these problems Polyani et al. (U.S. Pat. No. 4,050,450) proposed using a hollow tube as a calibrating device. Conceptually this proposal has merit since it inherently avoids contact of the catheter tip with a liquid.
One of the present inventors, however, has tested this technique and found it objectionably imprecise--that is, not adequately repeatable. We cannot account entirely for the observed imprecision, though it is likely related to the difference between reflection properties of a smooth plastic wall and reflection properties of a particle suspension in a liquid medium such as blood.
Shaw et al. (U.S. Pat. No. 4,322,164) have described a calibration system using reflecting particles suspended in a transparent solid polymeric medium as an optical calibration standard. A spring presses the surface of the polymer against the catheter tip.
In the Shaw system, a latch mechanism holds the calibration surface out of contact with the catheter tip until the time for calibration. Then the user unlocks the latch mechanism (by actuating a release device manually through a sterile envelope), allowing the spring to push the calibration surface across a short gap into contact with the catheter tip.
Shaw's innovation represents an important step forward in this technology. His suspension of particles in a transparent solid medium offers substantial promise of simulating quite closely the reflection properties of blood. Shaw, like Polyani, avoids using a liquid as the calibration standard.
Thus the Shaw patent presents an ingenious and very useful solution to a knotty problem. Nevertheless that solution has its own drawbacks--particularly with regard to the mechanics of use--and so leaves considerable room for refinement.
These drawbacks will first be enumerated, and then discussed in detail. First, the Shaw device is needlessly elaborate mechanically. Secondly, it virtually prevents prechecking the calibration or even the general integrity of the optics after packaging but before final use.
Thirdly, Shaw's device is somewhat awkward in use because it must be actuated through the sterile container. Fourthly, it is subject to measurement errors that can arise from this procedure.
Finally, it places potentially conflicting requirements on the physical properties of the suspension medium. We will now take up each of these points in turn.
First as to the mechanical elaborateness or complexity of the Shaw device, that complexity includes providing:
(a) a movable gripper that holds the catheter in place by friction (after it is positioned within the overall device),
(b) a separate movable plunger that carries the calibration surface into contact with the catheter,
(c) a mechanical track along which the plunger can move (and it must move reliably),
(d) a spring for impelling the plunger along the track to bring the calibration surface into contact with the catheter tip with a reliably predetermined force,
(e) a latch to prevent the plunger from moving until desired, and
(f) a rocker arm that increases the lateral grip on the catheter at the last instant before the plunger is allowed to move.
All this must be accomplished without compromising the light-tight character of the entire unit, and of course without materially increasing the cost of the catheter.
Turning secondly to the desirability of prechecking calibration (or even prechecking the general operability of the optical-fiber subsystem): the Shaw invention deters such prechecking because the latch-release mechanism is designed to "fire" just once.
It would be desirable to have a means of verifying continuing integrity of the catheter if it remains in storage for a long time in the manufacturer's warehouse. Such verification, within the manufacturer's premises, could even include verifying stability of calibration, since the same instrument could be used for initial and all subsequent checks.
It would also be desirable to have a means of verifying continuing integrity of the catheter if it remains in storage for a long time in an intermediate wholesaler's warehouse. Here again, stability of calibration could be verified within that facility.
It would be even more desirable to have a means of verifying the integrity of a catheter upon arrival in the storeroom of the hospital or research facility where it is to be used. In this way the usability of a stock of catheters could be guaranteed against the rigors of long-distance shipment.
It is only very minimally useful to conduct such verifications when the catheters are drawn out of the storeroom for use, for at that point a timely replacement may be impossible. It will be understood that stability of calibration could be checked in this context as well.
In principle, for later reuse Shaw's plunger could be pulled back out and the latch reseated. Such a procedure, however, would be tricky to perform through the sterile container--at least without compromising the positional accuracy of the catheter tip in the calibration device.
That brings us to the third drawback: awkwardness in use through the sterile container. The Shaw device must be actuated by pressing in on the rocker arm, to increase the gripping force on the catheter and simultaneously release the latch that restrains the spring-loaded plunger.
As a practical matter, however, "pressing in on the rocker arm" in this context means squeezing the portion of the device where the rocker arm is accessible. Otherwise the entire device will simply slide away from the user's finger.
In order to squeeze the device, one must grip it between thumb and forefinger. Depending upon the initial orientation of the device in its package, this may require either that the user somehow position the thumb or forefinger (working through the sterile container) beneath the device, or that the device be rotated in its shipping tray so that the direction of motion of the rocker is generally horizontal.
Therefore the user must be very nimble-fingered, or the device must be held on the tray by a formed mount (yet another elaboration, and one that would increase the potential for damage during shipment), or in preparation for squeezing the rocker the user must rotate the device with the other hand, again through the sterile container.
During any of these operations, of course, there is a constant risk of rupturing the container and thereby exposing the catheter to contamination. It is not our intention to make more of this awkwardness than there is, but it will be apparent that use of the Shaw device is not completely without pitfalls.
The fourth drawback mentioned above is the potential for measurement error arising from the cumbersome manipulation of the calibration device through the sterile container. This is a more complicated matter to discuss.
On one hand, there may be means for mitigating the awkwardness of operation. Such means may include a reasonably reliable preorientation of the device in its package, and/or extraordinary dexterity on the part of the user. These factors may "save" the Shaw device from the inherent awkwardness discussed above.
Furthermore, awkwardness in use is in a sense self-limiting. The user can determine clearly--by direct visual observation, coupled with taking a calibration reading--whether he or she has been successful in releasing the latch.
On the other hand, such a "save" may yet leave a drawback that is even more problematical, due to being hidden. The user in fumbling with the device to rotate it into position for operation, or in the actual step of releasing the latch, may inadvertently damage either the calibration device or the catheter itself in one way or another.
For example, the free pivoting of the rocker arm may be impaired, the catheter-gripping device may be squeezed too tightly against the catheter, the cylindrical track in which the plunger operates may be deformed, or the plunger after release may be pushed too hard against the catheter tip. Although these consequences may all be unlikely, what is very likely is that if they do occur they will not be detected, and they will significantly alter the conditions of calibration.
The intermediate result is a concealed and probably systematic error in calibration--that is to say, one that will persist even if the calibration reading is continued for a protracted period, or even if several such readings are taken over a period of hours. The final result can be a serious misdiagnosis that has catastrophic effects for, e.g., a heart patient.
The final drawback introduced earlier is the placement of possibly conflicting constraints on the physical properties of the suspension medium. Shaw's patent suggests at several points--including the abstract and the claims--that the material must be, e.g., "compliant at the surface 14 and noncompressible" (column 4, lines 5 and 6).
These potentially inconsistent requirements are elsewhere expressed thus:
The mass of the reference element 17 should exhibit compliant characteristics at least at the surface to assure intimate optical engagement of the surface 14 of the reference element 17 with the ends or apertures of the optical fibers that are exposed at the distal tip 231 [of] the catheter 12. The incompressible characteristic of the mass is desirable to prevent changes in concentration of the uniformly dispersed particles 36 within the mass.
Yet another expression of the constraints on the suspension is this: "a solid medium that has a substantially incompressible body which is sufficiently compliant at its surface for intimate contact with the end of the light guide".
At the outset it is unclear whether an optimal calibration medium is nonuniform, or at least nonisotropic, in its properties--or whether it is possible for an entirely homogeneous substance to satisfy the requirements.
Shaw does not explain how much compliance "at the surface" can be accommodated before "changes in concentration . . . within the mass" become excessive. Compliance, after all, is not truly a "surface" phenomenon but necessarily implicates the "body" or "mass" of the material.
Shaw does advise one skilled in the art to use "[s]ilicone resins which cure to a substantially transparent, compliant and incompressible solid mass" (emphasis added). This specification seems clearly to aggravate, rather than circumvent, the paradox just described.
Part of the dual requirement on Shaw's calibration medium arises from the dual way in which he uses the medium: first percussively, and then quantitatively. In other words, Shaw's catheter tip and calibration surface first must both survive the impact between them, and then are expected to act as well-behaved components of a high-precision measurement system.
Since it is not feasible in current technology to compromise the rigidity of the optical fibers in the catheter tip, all of the accommodation must be provided in the calibration surface.
If that surface were hard, then (1) upon impact it or the tip could crack or shatter, and (2) after impact it might not conform well to the optical surfaces to ensure a "liquid-like" optical engagement. On the other hand if the calibration mass were soft, then upon impact it could compress and throw off the calibration.
In this way of looking at things, the problem arises due to the impact, and one wonders whether it could be avoided simply by shipping the apparatus with the latch already released. Shaw's device, however, is plainly designed on the assumption that such a solution is unacceptable.
Otherwise the latch and release mechanisms could be simply omitted. The same is even more apparently true of his device that increases the lateral grip on the catheter body at the instant the release mechanism is triggered.
Shaw does not explain, and we can only speculate, whether he even thought of this solution, or if so then why he discarded it. One possibility is that Shaw was concerned about the effects of constant force on the catheter tip or the calibration medium, or both.
His device employs a spring to "urge" the calibration surface against the catheter tip. In very protracted pressing of the standard surface against the tip, either the mass of the calibration standard or the nonoptical bulk of the tip itself--the portion surrounding the fibers--would be likely to deform significantly.
The result could be problems of calibration or operation, or both kinds of problems. Anticipation of such problems is thus one possible reason for Shaw's "last minute" release mechanism and procedure. That mechanism and procedure, however, are precisely what produce the several drawbacks already pointed out.
The five problem areas just discussed all arise from a "blind spot" in the Shaw approach. That blind spot is essentially a natural inclination to emulate in a new hardware context the prior wet methods of calibration.
A more sophisticated approach would recognize that such emulation is no longer necessary and would free the hardware configuration from purely historical constraints that produce the noted drawbacks. Such a solution would of course be highly desirable.